1. Field of the Invention
This invention relates to a magnetic field generating system utilized for a magnetic resonance imaging apparatus.
2. Description of the Related Art
Conventional magnetic resonance imaging apparatus (referred to as MRI apparatus hereinafter) is arranged to generate a uniform static magnetic field, places a subject under examination in the static magnetic field, applies an RF (Radio Frequency) magnetic field in the direction perpendicular to the static magnetic field so that the magnetic resonance phenomenon occurs only within a predetermined slice of the subject, applies linear gradient magnetic fields after termination of the application of the RF field so as to acquire magnetic resonance signals (referred to as MR signals hereinafter) generated from atomic nuclei in the subject, and Fourier transforms the MR signals to reconstruct an image.
The MRI apparatus is equipped with a magnetic field generating device which generates, as shown in FIG. 1, the static magnetic field in the direction orthogonal to the body axis of subject P placed between a pair of permanent magnets 1 and 2 facing each other with a predetermined space therebetween. The magnetic field generating device uses "parallel 4 wires" to generate gradient magnetic fields in Z and X directions orthogonal to each other with the body axis of subject P taken in the Z direction. The parallel 4 wires shown in FIG. 1 are used for generating the gradient magnetic field in the X direction and include parts of coil 3, 4, 5, and 6 wound around permanent magnet 1 or 2 and iron yoke 7. That is, portions of coils 3, 4, 5, and 6 that locate on the opposed faces of permanent magnets 1 and 2 are arranged to be straight and parallel to each other, and these parallel coil portions 3a, 4a, 5a, and 6a form the parallel 4 wires. This secures a space in which subject P is placed. Each of coils 3, 4, 5, and 6 in FIG. 1 have three turns.
In such an arrangement as described above, since the coils are wound around iron yoke 7, the inductance of the coil becomes large, leading to the deterioration of rising characteristics of the gradient magnetic fields. Further, coil portions 3b, 4b, 5b, and 6b of the parallel 4 wires generate magnetic fields in directions opposed to those of predetermined gradient magnetic fields, so that the intensity of magnetic fields decreases and the linearity of the gradient magnetic fields deteriorates.
When a static magnetic field Bo is generated in the direction of the body axis of subject P as shown in FIG. 2, a gradient magnetic field in the Y direction can be generated by disposing coils 8a and 8b to the direction of the body axis. To obtain a predetermined magnetic field intensity, coils 8a and 8b may be provided with plural turns to the direction of the body axis of subject P like coils 9a and 9b as shown in FIG. 3. However, when static magnetic field Bo is generated in the direction perpendicular to the body axis of subject P as shown in FIG. 4, since permanent magnets 1 and 2 are disposed to the direction of the static magnetic field, coils wound to the direction of the static magnetic field would narrow a space into which subject P under examination is to be inserted. In particular, to generate gradient magnetic fields in the X, Y and Z directions, respective coils must be disposed within the space for inserting subject P.
In view of the above it is desired that the inductance of coils used for generating gradient magnetic fields can be small, gradient magnetic fields have sufficient intensity and linearity, and a sufficient space for a subject under examination can be secured.